The invention relates most generally to light-transmitting solid-state optical materials, more particularly to metal-ion-doped solid-state laser host crystals, and most particularly to neodymium- and ytterbium-doped vanadate crystals having the apatite structure, such as Sr.sub.5 (VO.sub.4).sub.3 F.
Solid-state lasers based on rare-earth-doped crystals are rapidly rising in technological significance, and these types of systems are becoming more commonly employed in industrial, medical, scientific, and military applications. The diverse nature of applications, however, require the availability of a wide variety of laser materials whose properties are appropriately matched to each technical scenario. Nd-doped Y.sub.3 Al.sub.5 O.sub.12 (Nd:YAG) is the most common laser material, owing to its high emission cross section, relatively long energy storage time, and robust thermomechanical properties. The usual output wavelength for Nd:YAG is 1.064 .mu.m, and it is pumped either with laser diodes at 0.808 .mu.m, or with a flashlamp source emitting a broad white light spectrum. In either case, Nd:YAG suffers from some difficulty in permitting an efficient and convenient means of being pumped, as a consequence of the narrow spectral width of its absorption lines.
Many new Nd laser materials have become available during the last decade which offer both advantages and disadvantages with respect to Nd:YAG. For example, Nd-doped LiYF.sub.4 (YLF) has wider absorption features and a longer energy storage time compared to Nd:YAG, although YLF is not as mechanically robust. Nd-doped phosphate glass, on the other hand, can be melted in large size at much lower cost, although the cross section is considerably lower than Nd:YAG. The Nd-doped YAlO.sub.3 (YALO) crystal exhibits minimal thermal birefringence, but is more difficult to grow. There exist numerous other examples of Nd-doped media that offer some type of trade-off in properties with respect to those of Nd:YAG. In particular, Nd-doped YVO.sub.4 possesses a remarkably high emission cross section that is larger than that of Nd:YAG, while nevertheless having a greater absorption linewidth. These two properties of Nd:YVO.sub.4 ease both the wavelength and power requirements of the diode pump source, for low power applications. Unfortunately, the YVO.sub.4 host is very difficult and expensive to grow as a single crystal, even in small sizes (&lt;1 cm).
Solid-state laser media can be based on many other laser ions in addition to Nd, and these alternative ions can be characterized by significantly different optical properties. For example, Yb lasers must be diode-pumped at longer wavelengths than Nd lasers, thereby requiring the use of InGaAs laser diodes (rather than AlGaAs as for Nd). Yb lasers are furthermore generally characterized by a reduced quantum defect between the pump and laser wavelengths, thereby leading to a reduction in the amount of heat deposited into the gain medium by the pump source.
British Patent Specification 1,215,279 describes single-phase solid solutions of a host crystal, calcium fluorovanadate, Ca.sub.5 (VO.sub.4).sub.3 F, in which a portion of the calcium has been substituted by a rare earth ion, and electroneutrality restored by the introduction of sodium ion or by substitution of oxygen ion for a portion of the fluorine. The compositions are useful in the polycrystalline powdered state as phosphors and in the single crystal state as laser crystals. The laser crystals are pumped by sources such as mercury vapor or xenon lamps.
L. H. Brixner, "Crystal Growth and Fluorescent Properties of Rare Earth-Doped Calcium Fluorovandate," J. Solid State Chem. 1, pp. 185-189, (1970), describes the growth of large single crystals with the apatite structure by the Czochralski method. The crystals are doped with various rare earths, with charge compensation using sodium or oxygen ions. The use as laser crystals is suggested.
L. H. Brixner et al., "Preparation and Properties of Some Strontium-Oxometallates," J. Solid State Chem. 2, pp. 55-60, (1970), describes the formation of a number of ternary strontium oxides by the flux reaction method.
British Patent Specification 946,073 describes halo-vanadates of the formula XMO-V.sub.2 O.sub.5 -YMH.sub.2 where M=Zn, Cd, Mg, Sr, or Ba, H=Cl or F, X is a number from 1 to 6, Y is a number from 0.5 to 1.0. These materials are luminescent materials useful in fluorescent electric discharge lamps.
British patent specification 1,197,316 describes a fluoroapatite (FAP) crystal, calcium fluorophosphate, Ca.sub.5 (PO.sub.4).sub.3 F with rare earth dopants, where the Ca may be substituted by Ba, Sr, or Mg and charge compensation for the rare earth dopant may be with an alkali metal or oxygen ions. The material is useful as a laser crystal which can be pumped by a flashlamp.
U.S. patent application Ser. No. 07/792,792 U.S. Pat. No. 5,280,492) filed Nov. 15, 1991, entitled "Yb:FAP and Related Materials, Laser Gain Medium Comprising Same, and Laser Systems Using Same," expands the fluorophosphate crystal with apatite structure to M.sub.5 (PO.sub.4).sub.3 X where M is Ca, Sr, Cd, Pb or a combination of monovalent and trivalent ions Li, Na, K or Rb, and Y, La, Gd, Lu or other rare earth and X is F, Cl, Br, I, OH, O or S. The Yb doped crystal is useful as a laser gain material and can be pumped with laser diodes at 0.905 or 0.98 microns.